Support plain serialization for alias of aliases

[nmiyake]

What happened?

• Define an alias type that resolves to another alias type that resolves to a primitive that defines PLAIN serialization (for example, rid)
• Generate Go code
• The generated code for the alias of the alias does not define a String() method, which means that PLAIN serialization fails (it outputs the struct form of rid instead of the string representation)

What did you want to happen?

• The generated code for the alias of the alias should generate a String() method that converts the receiver to the type it aliases and call String() on that type